Filter beds formed from one or more layers of filter media have been employed in a variety of known filters for filtering water or wastewater to remove impurities from liquids. For example, filter beds of granular media have been used in upflow filters, downflow filters as well as other types of filters including bi-flow filters. After the filter has been operating for a while, it is necessary to wash the filter bed to remove the impurities trapped in the filter bed during the filtration mode. Various methods have been used to wash the filter bed including but not limited to the steps of: (i) liquid only wash; (ii) air only wash; (iii) liquid and air concurrently; (iv) liquid only followed by air only; (v) air only followed by liquid only; and, (vi) liquid and air concurrently followed by liquid only.
It is important that the washing fluid is thoroughly distributed through the filter bed during the washing mode in order to remove an adequate amount of the impurities trapped in the filter bed during operation of the filter in the filtration mode. It is similarly important to uniformly distribute influent through the filter bed in-service fluids during the filtration mode. Various underdrains systems have been used in an attempt to distribute fluids uniformly throughout the filter bed.
Filters having a granular media filter bed above plenum style underdrains (e.g., Wheeler Bottoms, nozzle bottoms, strainers, Hydrocones or other similar underdrain types) are subject to maldistribution of the washing fluid during washing of the filter bed.
Maldistribution is often caused by pressure variations in the plenum induced by velocity gradients, friction loss, turbulence, and recirculation zones. The geometry of the plenum, the size and location of the backwash inlet, and the presence of piers or other elements that support the plenum style underdrain influence the hydraulic characteristics mentioned above. The plenum style underdrain typically uses fluid distributors (e.g., nozzles, strainers, Hyrdocones, inverted pyramidal depressions, etc.) that have identical hydraulic characteristics throughout the filter. Therefore, these types of underdrains cannot compensate for pressure variances in the plenum resulting in maldistribution of the washing fluid or influent in those instances where the washing fluid and/or influent passes from the plenum through the underdrain prior to entering the filter bed.